1. Field of the Invention
The present invention relates to a process of producing partially hydrolyzed polyacrylamide which has high solubility in water and can be widely used as, for example, a flocculant for water treatment, a size for paper manufacture, etc. More particularly, the invention relates to an improved process of producing partially hydrolyzed polyacrylamide by polymerizing a highly concentrated aqueous solution of acrylamide monomer in the presence of an alkali such as sodium hydroxide and boric acid to also partially hydrolyze the polymer formed substantially simultaneously with the polymerization.
2. Description of the Prior Art
Polymers prepared by homogeneous-polymerizing in an aqueous medium a monomer capable of forming a water-soluble polymer differ from each other in fluidity depending on the monomer concentration, degree of polymerization, and monomer composition employed for each polymer prepared. From a practical standpoint, acrylamide can be polymerized using various processes but an aqueous solution polymerization process which can be conducted economically and simply and can be used easily to produce a high molecular weight polymer suitable as a flocculant or a dispersing agent has now been employed in various fields. In a polymerization using a solvent or a dispersion medium as in solution polymerization, it is better that a high concentration monomer solution or polymer solution be used from the standpoints of the dimensions of the polymerization apparatus and solvent removal or drying of the polymer solution formed. For example, U.S. Pat. Nos. 3,477,932, and 3,558,759 and British Pat. No. 1,217,552 disclose polymerization of acrylamide at a high concentration in aqueous solution. Furthermore, from the standpoint of facilitating transportation and handling, a polymer powder has been required in place of a low-concentration liquid product which has hitherto been employed. However, polymerization in such a high-concentration monomer solution to meet such requirements is accompanied by other difficulties. For example, the viscosity of the polymer solution increases greatly.
In using polyacrylamide as a flocculant, it has been desired to establish a technique for producing a high molecular weight polyacrylamide having a desired hydrolysis percentage ranging from about 5 mol percent to about 50 mol percent since the flocculation ability of the polymer increases as the molecular weight of the polymer increases and further the suitable percentage of polyacrylamide hydrolyzed depends upon the properties of the suspended material to be flocculated.
In order to obtain a polymer powder by drying an aqueous solution of the polymer, it is obviously desirable to polymerize a monomer at a concentration as high as possible but as described above, polyacrylamide, in particular hydrolyzed polyacrylamide, tends to become quite a viscous liquid or to gel and the concentration of the polymer solution which can be treated as liquid substantially is less than from about 10 to about 15% by weight although this depends upon the degree of polymerization. In particular, for a high molecular weight polyamide having a molecular weight of about 5,000,000 which is usually used as a flocculant, a concentration of about 10% by weight frequently makes it difficult to handle the polymer composition as a liquid.
On the other hand, processes for preparing hydrolyzed polyacrylamide are generally known such as a process in which acrylamide is copolymerized with acrylic acid or a salt thereof and a process in which a homopolymer of acrylamide is prepared and then the homopolymer is hydrolyzed. However, the former process has the disadvantage that it is difficult to obtain a high molecular weight copolymer. Also, in the latter process, when the polymerization is conducted at a monomer concentration of higher than about 10% by weight and in particular higher than about 15% by weight, the polymer obtained is a viscous gel and thus if such a polymer is hydrolyzed in a post treatment (e.g., as disclosed in British Pat. No. 1,217,552), a homogeneous hydrolyzed polymer is not obtained.
Accordingly, in order to obtain a partially hydrolyzed polyamide in a high concentration, the employment of a so-called simultaneous polymerization-hydrolysis process is required wherein the hydrolysis reaction of polyacrylamide is conducted substantially simultaneously with the polymerization of the acrylamide.
The simultaneous polymerization-hydrolysis reaction of acrylamide is known and sodium hydroxide or ammonia is usually used as a hydrolyzing agent, for example, as disclosed in U.S. Pat. Nos. 2,820,777 and 3,200,098 and British Pat. No. 1,242,551. However, when such a hydrolyzing agent is used, a high molecular weight polymer is not obtained and thus various investigations have been made to overcome such a difficulty. For example, a method wherein an alkali metal carbonate is used as the hydrolyzing agent, for example, as disclosed in U.S. Pat. No. 3,022,279, a method wherein (1) an alkali metal hydroxide or an alkali metal carbonate along with (2) an alkali metal sulfate or an alkali metal chloride is used as hydrolyzing agent as disclosed in U.S. Pat. No. 3,414,552, and a method wherein sodium phosphate and sodium borate are used together as the hydrolyzing agent, for example, as disclosed in Japanese Pat. Publiciation No. 890/72, have been proposed.
However, some difficulties are encountered in the use of such hydrolyzing agents as proposed above when the monomer concentration is higher than about 15% by weight although satisfactory results can be obtained when the monomer concentration is comparatively low, for example, at about 10% by weight. For example, when an alkali metal carbonate is used, a water-insoluble polymer is formed to a great extent while when a phosphate and a borate are used together, the phosphate does not dissolve well in an aqueous solution of the monomer at a high concentration state due to the low solubility of the phosphate. These factors make it difficult to provide a highly hydrolyzed product.